Apparatuses that detect the approach of a moving component toward surrounding components, in order to prevent an imminent collision that might damage the components, are widely known. Contact switches, which are available in many configurations, may be used as an extremely simple embodiment.
DE 42 34 375 discloses an apparatus for protecting personnel and preventing collisions of driverless forklift vehicles, which comprises a light barrier between the tine tips of the lifting fork.
Unexamined Application DE 37 00 009 A1 discloses an optoelectronic safety zone apparatus for protecting moving or stationary objects from improper contact 0or collision, in which an optoelectronic distance sensor, which sweeps a light beam over an adjustable angular range, is used. By ascertaining the size of the light beam spot on surrounding objects, conclusions are drawn as to the distance; if it falls below a safe distance, a signal is outputted.
The known apparatuses have the disadvantage that initially they can sense relative approaches only from one direction. Only by pivoting the apparatus or by using several apparatuses oriented for different directions can approaches from different directions be sensed.
Particularly when additional pivoting or complex sensing and analysis of measured values (for example determination of a light spot size) is necessary, switching delays occur during which the undesired approach continues.
Contact switches can be configured in such a way that they have short switching travels. They have, however, the previously mentioned disadvantage that they sense approaches only from one direction. In addition, contact switches have a very large physical extension compared to the switching travel, in exactly that direction. Especially when contact switches must be used for all spatial directions, movement clearance is greatly restricted or additional space must be provided.
In an ultrahigh-accuracy coordinate measuring machine, a very heavy X/Y measurement stage made of Cerodur, sliding on an air bearing and carrying a specimen whose features are to be measured, is displaced with an accuracy of a few nanometers. With a measurement machine of this kind, for example, feature widths or feature spacings of a wafer exposure mask can be determined. The relative position of the X/Y measurement stage is measured interferometrically. An objective is arranged above the specimen, and a condenser below the specimen. The specimen can be examined microscopically in both an incident-light and a transmitted-light configuration. Contact sensors must be implemented in order to protect protruding components from, in particular, collisions with the heavy X/Y measurement stage.
The components of an ultrahigh-accuracy coordinate measuring machine of this kind are manufactured from very expensive materials that are difficult to machine.
The components, in particular the measurement stage, should therefore be no larger than necessary. A space-saving configuration must also be selected in view of the high cost of the clean-room space in which such measurement machines are installed. In this context, the use of contact sensors or contact switches that require a great deal of installation space is disadvantageous.